OBJECTIVES: Polyethylene glycol (PEG)-based gut lavage solutions are safe and effective, but require consumption of large volumes of fluid. We compared a new 2 L solution of PEG plus ascorbic acid (PEG + Asc) with standard 4 L PEG with electrolytes (PEG + E) for bowel cleansing before colonoscopy to determine efficacy, safety, and patient acceptability. METHODS: Consenting adult inpatients scheduled to undergo colonoscopy were randomized to receive either 2 L PEG + Asc or 4 L PEG + E. Preparations were taken as split doses the evening before colonoscopy and the following morning. The PEG + Asc group took 1 L at each administration (i.e., total dose of 2 L). The PEG + E group took 2 L at each administration (i.e., total dose of 4 L). Bowel cleansing success was assessed via videotapes by independent, blinded raters. Statistical noninferiority was predefined as a difference of <15% in the lower limit of the 97.5% confidence interval for treatment difference. Patient views on the preparations were elicited. Adverse events were noted. RESULTS: Successful gut cleansing was achieved in 136 of 153 (88.9%) cases of the PEG + Asc group and 147 of 155 (94.8%) cases of the 4 L PEG + E group (mean difference -5.9 [-12.0-infinity]). The difference fell within the predefined limit for noninferiority. Clinical and laboratory parameters showed no difference in safety profile. Patient ratings of acceptability and taste were better for the PEG + Asc group than for the PEG + E group (P < 0.025). CONCLUSIONS: The combination of ascorbic acid and PEG-based bowel preparation reduces the volume patients have to drink without compromising efficacy or safety. The low-volume PEG + Asc preparation was more acceptable to patients, and should, therefore, improve effectiveness in routine practice.